Non-thermal particle acceleration in collisionless relativistic electron-proton reconnection

Abstract: Magnetic reconnection in relativistic collisionless plasmas can accelerate particles and power high-energy emission in various astrophysical systems. Whereas most previous studies focused on relativistic reconnection in pair plasmas, less attention has been paid to electron-ion plasma reconnection, expected in black hole accretion flows and relativistic jets. We report a comprehensive particle-in-cell numerical investigation of reconnection in an electron-ion plasma, spanning a wide range of ambient ion magnetizations $\sigma_i$, from the semirelativistic regime (ultrarelativistic electrons but nonrelativistic ions, 0.001<<$\sigma_i$<<1) to the fully relativistic regime (both species are ultrarelativistic, $\sigma_i$>>1). We investigate how the reconnection rate, electron and ion plasma flows, electric and magnetic field structures, electron/ion energy partitioning, and nonthermal particle acceleration depend on $\sigma_i$. Our key findings are: (1) the reconnection rate is about 0.1 of the Alfvenic rate across all regimes; (2) electrons can form concentrated moderately relativistic outflows even in the semirelativistic, small-$\sigma_i$ regime; (3) while the released magnetic energy is partitioned equally between electrons and ions in the ultrarelativistic limit, the electron energy fraction declines gradually with decreased $\sigma_i$ and asymptotes to about 0.25 in the semirelativistic regime; (4) reconnection leads to efficient nonthermal electron acceleration with a $\sigma_i$-dependent power-law index, $p(\sigma_i) \simeq $const$+0.7 {\sigma_i}^{-1/2}$. These findings are important for understanding black hole systems and lend support to semirelativistic reconnection models for powering nonthermal emission in blazar jets, offering a natural explanation for the spectral indices observed in these systems.